This invention relates to the compositions of matter useful as a catalyst system, to a method for preparing these catalyst systems and to a method for polymerization utilizing the catalyst system.
The use of soluble Ziegler-Natta type catalysts in the polymerization of olefins is well known in the prior art. In general, such systems include a Group IV-B metal compound and a metal or metalloid alkyl cocatalyst, such as aluminum alkyl cocatalyst. More broadly, it may be said to include a mixture of a Group I-III metal alkyl and a transition metal complex from the Group IVB-VB metals, particularly titanium, zirconium, or hafnium with aluminum alkyl cocatalysts.
First generation cocatalyst systems for homogeneous metallocene Ziegler-Natta olefin polymerization, alkylaluminum chlorides (AlR.sub.2 Cl), exhibit low ethylene polymerization activity levels and negligible propylene polymerization activity. Second generation cocatalyst systems, utilizing methyl aluminoxane (MAO), raise activities by several orders of magnitude. In practice however, a large stoichiometric excess of MAO over catalyst ranging from several hundred to ten thousand must be employed to have good activities and stereoselectivities. Moreover, it has not been possible to isolate characterizable metallocene active species using MAO. The third generation of cocatalyst, B(C.sub.6 F.sub.5).sub.3, proves to be far more efficient while utilizing a 1:1 catalyst-cocatalyst ratio. Although active catalyst species generated with B(C.sub.6 F.sub.5).sub.3 are isolable and characterizable, the anion MeB(C.sub.6 F.sub.5).sub.3.sup.- formed after Me.sup.- abstraction from metallocene dimethyl complexes is weakly coordinated to the electron-deficient metal center, thus resulting in a decrease of certain catalytic activities. The recently developed B(C.sub.6 F.sub.5).sub.4.sup.- types of non-coordinating anions exhibit some of the highest reported catalytic activities, but such catalysts have proven difficult to obtain in the pure state due to poor thermal stability and poor crystallizability, which is crucial for long-lived catalysts and for understanding the role of true catalytic species in the catalysis for the future catalyst design. Synthetically, it also takes two additional steps to prepare such an anion than for the neutral organo-Lewis acid.
Ligand modifications have played a key role in developing new "single-site" group 4 metallocene catalyst precursors for optimizing polymerization activity as well as polymer properties such as stereoregularity, molecular weight, thermal/rheological characteristics, bulky and polar comonomer incorporation and microstructure. In particular, complexes of bifunctional monocyclopentadienyl ligands having an appended heteroatom donor attracted considerable attention, as exemplified by "constrained geometry catalysts" having the formula ME.sub.2 Si(.eta..sup.5 -Me.sub.4 C.sub.5)(BuN)MX.sub.2 (CGCMX.sub.2 ; M=Ti, Zr, Hf; X=Cl, Me, CH.sub.2 Ph). These catalysts have a covalently attached amide donor ligand which stabilizes the electrophilic metal center electronically, while the short Me.sub.2 Si&lt;bridging group considerably opens the metal coordination sphere vis-a-vis a conventional metallocene. The result upon activation with a variety of cocatalysts is a new generation of catalysts which, among other features, efficiently produce ultra-low density elastomeric ethylene-octene copolymers. ##STR1##
Given the import of the Cp-appended heteroatom donor groups on the catalytic performance of such complexes, ligand design remains a very active and challenging area of olefin polymerization research and much attention has been paid to the design of new N- and O-containing ligands.